This invention relates to transmission controls for multispeed transmissions, and more particularly to electro-hydraulic controls, wherein valving members are incorporated to selectively establish a predetermined speed ratio in a transmission if an electrical malfunction occurs.
The use of electro-hydraulic controls for multispeed transmissions is becoming more wide spread. When these controls are installed in a six speed automatic transmission, it is common practice to have the transmission be selectively conditioned for the fourth speed ratio if unwanted electrical discontinuance should occur. Such devices have been generally satisfactory. However, there are some conditions under which a shift to the fourth speed ratio may not be desired. For example, when operating in either the lowest speed ratio or highest speed ratio, a shift to the fourth speed ratio may result in less tractive effort than required, or more engine speed than desired, respectively.
To prevent the transmission from shifting from the sixth ratio to the fourth ratio, corresponding U.S. application Ser. No. 230,938, filed Aug. 11, 1988, utilizes an inhibitor valve to prevent shifting from the sixth ratio to the fourth ratio if an electrical malfunction occurs. Another device disclosed in co-pending U.S. application Ser. No. 230,813, filed Aug. 11, 1988, incorporates a manual valve in the control mechanism to permit the operator to manually select and establish forward, low, reverse and neutral speed ratios in the transmission if an electrical malfunction should occur. Both of these solutions are satisfactory and will accomplish the intended purpose of the devices; that is, a "limp home" feature is incorporated into the transmission.